Position label of curved arrow in chemfig?

Question

I'm trying to create the following image in LaTeX:

However, I'm having trouble positioning the label on the second curved arrow. Below is what I have been able to generate.

If you'll notice that the second O2 is not exactly spaced like the first O2. Below is my MWE:

\documentclass{article}
\usepackage{chemfig}
\usepackage{fixltx2e}   % needed on my machine for textsubscript
\begin{document}
\begin{center}
\schemestart
\chemfig{*6(-=-=(-CH_3)-=)}
\arrow{-U>[O\textsubscript{2}][][][.5][]}
\chemfig{*6(-=-(-OH)=(-CH_3)-=)}
\arrow{-U>[*{0.north east}O\textsubscript{2}][][][-.5][-60]}
\chemfig{*6(-=(-OH)-(-OH)=(-CH_3)-=)}
\schemestop
\end{center}
\end{document}

I'm compiling using pdflatex mwe.tex using pdfTeX 3.1415926502.6.1.40.15 (TeX Live 2015/dev/Debian)

Answer 1

TL;DR: If you just want the solution, scroll down to after the line break.

Defining a new -u> arrow

From what I can see, there's not really an easy way to place the label for the -U> arrow at the bottom without a redefinition, shown below (place this in your preamble):

\makeatletter
\definearrow5{-u>}{%
    \CF@arrow@shift@nodes{#3}%
    \expandafter\draw\expandafter[\CF@arrow@current@style](\CF@arrow@start@node)--(\CF@arrow@end@node)node[midway](uarrow@arctangent){};%
    \CF@ifempty{#4}
    {\def\CF@uarrow@radius{0.333}}
    {\def\CF@uarrow@radius{#4}}%
    \CF@ifempty{#5}%
    {\def\CF@uarrow@absangle{60}}
    {\pgfmathsetmacro\CF@uarrow@absangle{abs(#5)}}
    %
    \edef\CF@tmp@str{[\CF@ifempty{#1}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,start angle=\CF@arrow@current@angle+90,delta angle=\CF@uarrow@absangle]node(uarrow@start){};
    %
    \edef\CF@tmp@str{[\CF@ifempty{#2}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,%
    start angle=\CF@arrow@current@angle+90,%
    delta angle=-\CF@uarrow@absangle]%
    node(uarrow@end){};
    \pgfmathsetmacro\CF@tmp@str{\CF@uarrow@radius*cos(\CF@arrow@current@angle)<0?"+":"-"}%
    \ifdim\CF@uarrow@radius pt>\z@
    \CF@arrow@display@label{#1}{0}\CF@tmp@str{uarrow@start}{#2}{1}\CF@tmp@str{uarrow@end}%
    \else
    \CF@arrow@display@label{#2}{0}\CF@tmp@str{uarrow@start}{#1}{1}\CF@tmp@str{uarrow@end}%
    \fi
}
\makeatother

This is based on the original -U> definition, but for labels below the arrow. Here, the syntax is (note the use of small letter u, instead of U):

\arrow{-u>[#1][#2][#3][#4][#5]}

Like the original, it takes 5 arguments, as follows:

• #1: label to be placed 'first' (arrow start)
• #2: label to be placed 'second' (arrow end)
• #3: yshift for the arrow, positive for upwards shift, vice versa
• #4: radius of arc (default 0.333)
• #5: angle for arc (default 60)

Just to show it works for all 5 arguments:

\documentclass{article}
\usepackage{chemfig}
%\usepackage{fixltx2e}   % needed on my machine for textsubscript

\makeatletter
\definearrow5{-u>}{%
    \CF@arrow@shift@nodes{#3}%
    \expandafter\draw\expandafter[\CF@arrow@current@style](\CF@arrow@start@node)--(\CF@arrow@end@node)node[midway](uarrow@arctangent){};%
    \CF@ifempty{#4}
    {\def\CF@uarrow@radius{0.333}}
    {\def\CF@uarrow@radius{#4}}%
    \CF@ifempty{#5}%
    {\def\CF@uarrow@absangle{60}}
    {\pgfmathsetmacro\CF@uarrow@absangle{abs(#5)}}
    %
    \edef\CF@tmp@str{[\CF@ifempty{#1}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,start angle=\CF@arrow@current@angle+90,delta angle=\CF@uarrow@absangle]node(uarrow@start){};
    %
    \edef\CF@tmp@str{[\CF@ifempty{#2}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,%
    start angle=\CF@arrow@current@angle+90,%
    delta angle=-\CF@uarrow@absangle]%
    node(uarrow@end){};
    \pgfmathsetmacro\CF@tmp@str{\CF@uarrow@radius*cos(\CF@arrow@current@angle)<0?"+":"-"}%
    \ifdim\CF@uarrow@radius pt>\z@
    \CF@arrow@display@label{#1}{0}\CF@tmp@str{uarrow@start}{#2}{1}\CF@tmp@str{uarrow@end}%
    \else
    \CF@arrow@display@label{#2}{0}\CF@tmp@str{uarrow@start}{#1}{1}\CF@tmp@str{uarrow@end}%
    \fi
}
\makeatother

\begin{document}
    \begin{center}
        \schemedebug{true} % Only to show boxes, nodes etc.
        \schemestart
        \chemfig{*6(-=-=(-CH_3)-=)}
        \arrow{-U>[\chemfig{O_2}][][][.5][]}
        \chemfig{*6(-=-(-OH)=(-CH_3)-=)}
        \arrow{-u>[A][B][5pt][.5][75]} % <---------- applying the new arrow type here
        \chemfig{*6(-=(-OH)-(-OH)=(-CH_3)-=)}
        \schemestop
    \end{center}
\end{document}

yields:

Note how:

• A and B are placed at start and end of the arrow (in that order), as specified.
• I indicated a positive yshift value, and indeed, the arrow was shifted upwards. (compare the red node points and the white node points of the arrow)
• The radius of the arc is also visibly longer because 0.5 > 0.333 of the default.
• The angle of the arc can be seen to be larger than the default because 75 > 60 (change it to other values to see for yourself).

---

Your Solution

\documentclass{article}
\usepackage{chemfig}
%\usepackage{fixltx2e}   % needed on my machine for textsubscript

\makeatletter
\definearrow5{-u>}{%
    \CF@arrow@shift@nodes{#3}%
    \expandafter\draw\expandafter[\CF@arrow@current@style](\CF@arrow@start@node)--(\CF@arrow@end@node)node[midway](uarrow@arctangent){};%
    \CF@ifempty{#4}
    {\def\CF@uarrow@radius{0.333}}
    {\def\CF@uarrow@radius{#4}}%
    \CF@ifempty{#5}%
    {\def\CF@uarrow@absangle{60}}
    {\pgfmathsetmacro\CF@uarrow@absangle{abs(#5)}}
    %
    \edef\CF@tmp@str{[\CF@ifempty{#1}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,start angle=\CF@arrow@current@angle+90,delta angle=\CF@uarrow@absangle]node(uarrow@start){};
    %
    \edef\CF@tmp@str{[\CF@ifempty{#2}{draw=none}{\unexpanded\expandafter{\CF@arrow@current@style}},-]}%
    \expandafter\draw\CF@tmp@str (uarrow@arctangent)%
    arc[radius=\CF@compound@sep*\CF@current@arrow@length*\CF@uarrow@radius,%
    start angle=\CF@arrow@current@angle+90,%
    delta angle=-\CF@uarrow@absangle]%
    node(uarrow@end){};
    \pgfmathsetmacro\CF@tmp@str{\CF@uarrow@radius*cos(\CF@arrow@current@angle)<0?"+":"-"}%
    \ifdim\CF@uarrow@radius pt>\z@
    \CF@arrow@display@label{#1}{0}\CF@tmp@str{uarrow@start}{#2}{1}\CF@tmp@str{uarrow@end}%
    \else
    \CF@arrow@display@label{#2}{0}\CF@tmp@str{uarrow@start}{#1}{1}\CF@tmp@str{uarrow@end}%
    \fi
}
\makeatother

\begin{document}
    \begin{center}
%       \schemedebug{true} % Activate if you want to see the nodes etc.
        \schemestart
        \chemfig{*6(-=-=(-CH_3)-=)}
        \arrow{-U>[\chemfig{O_2}][][][.5][]}
        \chemfig{*6(-=-(-OH)=(-CH_3)-=)}
        \arrow{-u>[\chemfig{O_2}][][][.5][]}
        \chemfig{*6(-=(-OH)-(-OH)=(-CH_3)-=)}
        \schemestop
    \end{center}
\end{document}

A few comments:

• I used \chemfig{O_2} instead of the \textsubscript version in your original code.
• Only other optional argument passed was 0.5 for the arc radius, which was in your original question. The rest can be left blank. Note the angle need not be specified if you just need it to be 60 degrees because it is the default value.